This invention relates generally to a pressure relief vent for a pressure vessel for containing a pressurized fluid and more particularly this invention relates to a new and improved jet-flow pressure relief vent particularly useful for relieving or venting an over-pressurized pressure vessel such as an over the road tank trailer.
As known to those skilled in the art, an over the road tank is a mobile pressure vessel typically pulled behind a truck or tractor and which tank is for containing a fluid, gas or liquid, during transportation from one point to another. Such contained fluids can be pressurized fluids or unpressurized fluids, however, the un-pressurized fluids may become pressurized due to heating of the tank by the sun which in turn heats the fluid contained in the tank causing it to be pressurized. Also, during an emergency situation, such as upon the over the road tank becoming heated in a fire, the fluid contained in the tank can become heated and pressurized.
As further known to those skilled in the art, over the road tanks are not formed in one piece but instead are typically formed from a plurality of individual pieces of metal welded together into the shape of an over the road tank. The manufacturer of the over the road tank manufactures the tank to what is known in the art as a working pressure which is determined by the thickness of the tank material, the strength of the welds, etc. Typically an appropriate safety factor is provided for the working pressure.
As is still further known to those skilled in the art, the Department of Transportation of the United States Government (DOT) promulgates regulations controlling over the road tanks. More specifically, the DOT promulgates regulations concerning the relieving or venting of the tank in the event of over pressurization of the contained fluid to prevent tank rupture and injury to persons and property. Such DOT regulations include a set pressure which is a pressure at which the pressure relief vent must begin venting the tank to relieve over-pressurization; such set pressure is related or proportional to the tank working pressure. As is still further known, the DOT also promulgates regulations concerning the rate at which the pressure relief vent must allow the over-pressurized fluid to flow out of the pressure relief vent at a pressure known as the flow pressure; this rate is called the flow rate and the flow pressure is a pressure greater than the set pressure by a predetermined amount.
The typical prior art pressure relief vent for an over the road tank trailer includes a housing mounted on the top of the tank trailer and which housing provides a valve seat at its top which is normally engaged by a poppet forced into sealing engagement with the valve seat by a compression spring mounted in the housing and providing downwardly acting force on the poppet; the underside of the poppet is exposed to the pressurized fluid in the tank through the housing and the pressurized fluid in the tank provides an upwardly acting force on the underside of the poppet. As is still further known to those skilled in the art, the compression spring is and preloaded such that the downwardly acting force provided by the spring on the poppet is sufficient to maintain the poppet in sealing engagement with the valve seat up to the set pressure in opposition to any upwardly acting force applied to the underside of the poppet by the pressurized fluid. As is still further known, the compression spring must be chosen such that upon the fluid in the over the road tank trailer becoming over-pressurized to the flow pressure the compression spring will compress sufficiently upwardly to permit the poppet to be lifted up off of the valve seat by the force created by the over-pressurized fluid a distance sufficient to provide a space or opening, typically an annular space or opening, between the valve seat and the poppet sufficiently large to permit the over-pressurized fluid to flow or escape therethrough as a stream, or streams, of over-pressurized fluid and at the required flow rate.
As is still further known, compression springs are characterized by what is known as a spring constant which is the relationship between the distance the spring will compress upon the application thereto of a given force. Accordingly, as is still further known, the spring constant for the compression spring of a pressure relief vent is the difference between the flow pressure and the set pressure in pounds per square inch times the area of the poppet in square inches, divided by the distance in inches between the valve seat and the poppet required to provide the above-noted flow rate. A general characteristic of compression springs, as is known, is that the higher the spring constant the stiffer the spring.
As is still further known, the DOT requires that the pressure relief vent for an over the road tank trailer be mounted on the tank trailer as close as possible to the top center of the tank trailer so that the pressure relief vent is exposed to the vapor space inside the tank trailer, with such vapor space, as is known, being the space between the fluid contained in the tank trailer and the top of the tank trailer. As is further known the height of the pressure relief vent is determined largely by the height or length of the compression spring.
Since the pressure relief vent must be mounted on the top center of the over the road tank trailer, it is obviously desirable, if not virtually required, that the height of the pressure relief vent be as small as-possible to permit the over the road tank trailer with the pressure relief vent mounted on top to safely pass under overhead structures such as bridges and the like with adequate clearance while maintaining the required flow rate. This means that the length or height of the compression spring of the pressure relief vent must be as small as possible and yet the compression spring, as noted above, must be sufficiently stiff to maintain the poppet in sealing engagement with the valve seat at the set pressure and which spring must be sufficiently un-stiff that it will compress sufficiently at the flow pressure to permit the poppet to be moved away from the valve seat a distance sufficient to provide the above-noted flow rate. Obviously, as known to those skilled in the art, these two spring requirements are in conflict with each other because, generally speaking and as is further known, the higher or longer the compression spring the lower the spring constant and the shorter the spring the higher the spring constant.
Numerous pressure relief vents are known to the prior art for relieving an over-pressurized pressure vessel such as the above-noted over the road tank trailer. Generally, the typical poppet of the prior art pressure relief vent is flat and has a flat underside against which the pressure in the pressure vessel, such as an over the road tank trailer, acts to apply or develop a force which lifts the poppet off of the valve seat. In these prior art pressure relief vents, the pressure acting against the underside of the poppet typically acts only against a flat poppet underside. However, other prior art pressure relief vents are known to the art wherein the poppet is provided with a shape other than flat, or at least other than flat at its outer peripheral underside portion,.which causes the vented or escaping over-pressurized fluid to develop an additional upwardly acting force for assisting the force applied to the central underside portion of the poppet by the over-pressurized fluid in lifting the poppet .off of the valve seat. Such assistance permits the use of a stiffer and therefore shorter in length compression spring for performing both the foregoing functions of sealing the poppet to the valve seat to the set pressure and compressing sufficiently to provide the flow rate at the flow pressure; such shorter in length compression spring permits the pressure relief vent to be shorter in height. An example of such prior art pressure relief vent is shown in FIG. 1.
The prior art pressure relief vent shown in FIG. 1 is identified by general numerical designation 10 and includes a housing 12 including an upper housing member 11 mounted threadedly to a lower housing member 13 in turn mounted threadedly to the top of a pressure vessel such as an over the road tank trailer 14. A compression spring, or more particularly a plurality of concentric compression springs 16 and 18, are mounted in the housing 12 and apply downwardly acting force indicated by arrows 19 and 20 which act against the top of a poppet 22 to maintain the poppet in sealing engagement with the valve seat 24 up to the set pressure. Upon occurrence of over-pressure in the tank trailer 14 greater than the set pressure, such over-pressure applies or develops force indicated by arrow 26 which acts on the underside of the poppet 22 and lifts the poppet 22 upwardly off of the valve seat 24 to permit the over-pressurized fluid to escape or vent as a stream of pressurized fluid passing between the space or area between the underside of the poppet 22 and the valve seat 24 and out of the housing 12 through the vent openings 28. It will be noted from FIG. 1 that the underside of the outer peripheral portion of the poppet 22 is provided with an inwardly and upwardly inclined surface 30 against which the escaping stream of over-pressurized fluid will also act and which inclined surface 30 will provide an upwardly acting component of force which will assist the force 26 acting against the central underside portion of the poppet 22 in lifting the poppet 22 off of the valve seat 24. However, it has been found that such inclined surface 30 does not provide a sufficiently high additional force to permit the height or length of the compression springs 16 and 18 to be sufficiently small or short in length to provide a pressure relief vent 10 sufficiently short in height as is desired for mounting of the pressure relief vent 10 on the top of an over the road tank trailer.
Accordingly, there exists a need in the pressure relief vent art for a pressure relief vent which is as short in height as possible and yet which pressure relief vent also satisfies the above-noted DOT regulations. This means, generally, that the pressure relief vent must include a compression spring as short in height as possible, and yet which compression spring both maintains the poppet in engagement with the valve seat at the set pressure and which compression spring compresses sufficiently at the flow pressure to permit the poppet to move away from, or be lifted off of, the valve seat a distance sufficient to provide sufficiently large space or area between the valve seat and the poppet to permit the over-pressurized fluid to flow therethrough at a rate at least equal to the flow rate.